Pump preload index/indicator

ABSTRACT

A method of estimating an amount of work available to be performed by a blood pump implanted in a patient includes calculating a first coordinate value characterizing a volume of blood impelled in the pump and a second coordinate value characterizing a differential pressure across the pump for each of a plurality of flow rate data points of a given cardiac cycle of the patient, each flow rate data point indicative of a flow rate of blood through the pump. An area enclosed by the first and second coordinate values of the plurality of flow rate data points is determined, the determined area being indicative of an amount of work available to be performed by the blood pump.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.15/424,057, filed Feb. 3, 2017, entitled PUMP PRELOAD INDEX/INDICATORand is related to and claims priority to U.S. Provisional PatentApplication Ser. No. 62/291,123, filed Feb. 4, 2016, entitled PUMPCAPACITY WORK INDEX, the entirety of which is incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

n/a

TECHNICAL FIELD

The present invention relates to methods and devices for estimating anamount of work being performed by the heart of a patient when the heartis operating in parallel with an implantable blood pump, and/or anamount of work available to be performed by the implantable blood pumpoperating in parallel with the heart.

BACKGROUND

An implantable blood pump used as a mechanical circulatory supportdevice or “MCSD” includes a pumping mechanism to move blood. The pumpingmechanism may be a radial flow pump, such as the HVAD® Pump manufacturedby HeartWare Inc. in Miami Lakes, Fla., USA. The HVAD® Pump is furtherdiscussed in U.S. Pat. No. 8,512,013, the disclosure of which is herebyincorporated herein in its entirety. Alternatively, the pumpingmechanism may be an axial flow pump, such as the MVAD® Pump, alsomanufactured by HeartWare Inc., and the pumps described in U.S. Pat.Nos. 7,972,122, 8,007,254 and 8,419,609, the disclosures of which arealso hereby incorporated herein in their entirety, or any other pumpsuitable for providing vascular assistance. In operation, the blood pumpdraws blood from a source such as the right ventricle, left ventricle,right atrium, or left atrium of a patient's heart and propels the bloodinto an artery such as the patient's ascending aorta or peripheralartery. Due to the nature of the application, the pumping mechanism mustbe highly reliable. Patient comfort is also a significant consideration.In addition to the pumping mechanism, the device may include acontroller and the drive electronics for the pumping mechanism. Thecontroller and drive electronics may receive power from an externalpower source. That power may be used to drive a motor of the pumpingmechanism at a desired speed.

In some cases, the blood pump may provide only partial support to thepatient. In such cases, the patient's heart continues to pump blood fromthe left ventricle to the aorta through the aortic valve (or, in thecase of the right ventricle, to the pulmonary artery through thepulmonic valve), and the blood pump further assists the activity of thepatient's heart in parallel. Although the heart only pumps blood intothe aorta during systole, the blood pump works during both systole anddiastole, when the aortic valve is open or closed. Thus, over a givenperiod of time including systole and diastole, the patient's heart andthe blood pump may each be responsible for some of the work performed topump blood to the patient's arteries.

It is generally understood that increasing the speed of the blood pumpcauses the pump to perform more work. In some cases, increasing thespeed of the pump may be beneficial for the patient, by allowing thepump to perform additional work in tandem with the heart. However, inother cases, the pump may already be operating at a speed at which thereis little or no additional work to be performed (e.g., blood to bepumped from the ventricle), in which case increasing patient's heart maysimply cause the patient's heart to perform less work, but notnecessarily increase the overall work performed. In some cases,increasing motor speed of the blood pump may leave so little work forthe heart that the aortic valve is not forced open during systole,thereby transitioning the pump from a state of partial-assistance to astate of full-assistance. Such changes in work performed by the heartmay be unwanted. Therefore, it is desirable to determine how much workis being performed by each of the heart and the blood pump, so that itmay further be determined whether it is desirable to increase a motorspeed of the pump.

Conventionally, the amount of work performed by the each of thepatient's heart and the blood pump may be determined based on invasivemeasurements. For example, ventricular work could be assessed usingcatheter-based measurements. The catheter-based measurements could beused to construct a pressure-volume loop (“PV loop”) indicating thetotal work performed by the patient's heart. FIG. 1 is a diagram of anexample PV loop, showing the volume of blood stored in the patient'sleft ventricle (“LVV” horizontal axis, measured in microliters) and thepressure exerted by the left ventricle (“LVP” vertical axis, measured inmmHg) over the course of a single cardiac cycle. In the example of FIG.1, stroke work (“SW”) exemplifies the amount of work performed by theheart. The potential energy (“PE”) may be considered an indication ofthe amount of work not performed by the heart, and therefore remainingfor the pump to perform.

The above example demonstrates catheter-based measurements for the“left” half of the heart. Similar measurements may be taken for the“right” half of the heart (e.g., right ventricular volume, rightventricular pressure, pulmonary pressure, etc.).

Invasive measurements may also be used to detect a situation in whichthe blood pump transitions from a state of partial-assistance to a stateof full-assistance. For example, catheter-based measurements of leftventricular pressure (“LVP”) and aortic pressure (“AOP”) may be used toidentify aortic valve closure during systole. If LVP and AOPmeasurements cross over one another during the course of the patient'scardiac cycle (particularly during the transitions between systole anddiastole), the cross over is an indication that the aortic valve hasopened, thereby causing AOP and LVP to be relatively the same (ascompared to during diastole). By contrast, if LVP and AOP do not crossover, that is an indication that the aortic valve has not opened evenduring systole, since LVP remains below AOP. However, it is impracticalto monitor the patient's heart and pump function invasively after theblood pump has already been implanted, particularly while the patient isoutside of a clinic or hospital.

SUMMARY

The present disclosure provides systems and methods for determining anamount of work being performed by a blood pump, and more particularly anamount of additional work available for a blood pump to perform. Thesystems and methods described herein are beneficial for partialassistance blood pumps, in which the blood pump is only partiallyresponsible for pumping blood to the patient's arteries, therebyproviding assistance to the work being performed by the patient's heart,as well as for full assistance blood pumps, in which all blood pumpedfrom a given ventricle to its corresponding arteries travels through thepump.

Work may be characterized as an amount of force applied to an object (inthis case blood) to move that object a given distance. Since thepressure exerted by the heart is equivalent to the force over thecross-sectional area that the force is applied, the work performed (orto be performed) by the heart may be characterized as the pressureexerted by the heart on the blood multiplied by the cross-sectional areathat the pressure is exerted, further multiplied by the distance thatthe blood is pushed.

Aside from the work performed by the heart, the work not performed bythe heart (and thereby left available to be performed by a pumpconnected thereto) may be similarly characterized. In the case of suchwork, the work may be thought of as a pressure component of the pump(e.g., differential pressure across the pump) multiplied by a volumecomponent (e.g., a flow rate of blood flowing through the pump, aderivative thereof, etc.).

Available pump work may be estimated, calculated or otherwise determinedover the course of one or more cardiac cycles of the patient. Notably,over the course of a cardiac cycle, the pressure and/or volume componentof the pump does not necessarily remain constant. Therefore, in suchcircumstances, the pump work determination should take into account thechanges in the pressure and volume components. The present disclosurefurther provides ways for these changes in the pressure and volumecomponents to be tracked and further integrated into a determination ofoverall available pump work.

Since in a partial-support blood pump the heart and pump both performwork, the work available to be performed by the blood pump may also bean indication of how much work the heart is performing. For example, ifthe pump is performing a relatively high amount of work, thereby leavingrelatively little available work to be performed, the heart is likelyalso performing a relatively low amount of work. Conversely, if the pumpis performing a relatively low amount of work, thereby leaving moreavailable work to be performed, the heart is likely performing arelatively high amount of work.

In one embodiment, a method of estimating an amount of work available tobe performed by a blood pump implanted in a patient includes calculatinga first coordinate value characterizing a volume of blood impelled inthe pump and a second coordinate value characterizing a differentialpressure across the pump for each of a plurality of flow rate datapoints of a given cardiac cycle of the patient, each flow rate datapoint indicative of a flow rate of blood through the pump. An areaenclosed by the first and second coordinate values of the plurality offlow rate data points is determined, the determined area beingindicative of an amount of work available to be performed by the bloodpump.

In another aspect of this embodiment, the method further includedetermining a starting time of the given cardiac cycle based on thedetermined flow rate data points crossing a running average flow rateand determining an ending time of the given cardiac cycle based on thedetermined flow rate data points crossing a running average flow rate,the plurality of flow rate data points of the given cardiac cycle beingindicative of a flow rate of blood between the starting time and endingtime.

In another aspect of this embodiment, determining the starting andending times are further based on one of: identifying consecutiveinstances of the determined flow rate data points crossing the runningaverage flow rate with a negative slope; identifying consecutiveinstances of the determined flow rate data points crossing the runningaverage flow rate with a positive slope; and identifying threeconsecutive instances of the determined flow rate data points crossingof the running average flow rate, the first of the three consecutiveinstances being the starting time and the third of the three consecutiveinstances being the ending time.

In another aspect of this embodiment, for each of the flow rate datapoints of the given cardiac cycle of the patient, the calculated firstcoordinate value is a derivative of the flow rate data point.

In another aspect of this embodiment, for each of the flow rate datapoints of the given cardiac cycle of the patient, the calculated secondcoordinate value is a differential pressure corresponding to the flowrate data point.

In another aspect of this embodiment, the method further includesdetermining a rotational speed of the blood pump, wherein the secondcoordinate value is determined at least in part using the determinedrotational speed of the blood pump.

In another aspect of this embodiment, the calculated second coordinatevalue is interpolated from a reference curve correlating differentialpressure across the pump and flow rate through the pump for a givenrotational speed of the blood pump.

In another aspect of this embodiment, the flow rate data points aredetermined based on a non-invasive estimation of flow rate.

In another embodiment, a method of controlling a partial-support bloodpump implanted in a patient includes calculating a first coordinatevalue characterizing a volume of blood impelled in the pump and a secondcoordinate value characterizing a differential pressure across the pumpfor each of a plurality of flow rate data points of a given cardiaccycle of the patient, each flow rate data point indicative of a flowrate of blood through the pump. An area enclosed by the first and secondcoordinate values of the plurality of flow rate data points isdetermined, the determined area being indicative of an amount of workavailable to be performed by the blood pump. The determined area iscompared to a predetermined value indicative of a maximum allowableamount of available work for which partial-support is maintained. Aspeed of operation for a motor of the blood pump is increased when thedetermined area is greater than the predetermined value.

In another aspect of this embodiment, the partial-support blood pump isa partial-support ventricular assist device for at least one of the leftand right ventricle of the patient, and wherein the predetermined valueis indicative of closure of at least one of the aortic and pulmonaryvalve of the patient during systole.

In yet another embodiment, a method of detecting an incipient suctioncondition at a blood pump implanted in an patient includes calculating afirst coordinate value characterizing a volume of blood impelled in thepump and a second coordinate value characterizing a differentialpressure across the pump for each of a plurality of flow rate datapoints of a given cardiac cycle of the patient, each flow rate datapoint indicative of a flow rate of blood through the pump. An areaenclosed by the first and second coordinate values of the plurality offlow rate data points is determined, the determined area beingindicative of an amount of work available to be performed by the bloodpump. The determined area is compared to a predetermined valueindicative of a maximum allowable amount of available work for whichblood fills the patient's heart. When the determined area is less thanthe predetermined value, determining the presence of an incipientsuction condition.

In another aspect of this embodiment, in response to determining thepresence of an incipient suction condition, automatically decreasing aspeed of operation of a motor of the blood pump.

In yet another embodiment, a method of detecting a blockage in a bloodpump implanted in a patient includes at a first speed of a motor of theblood pump, calculating a first coordinate value characterizing a volumeof blood impelled in the pump and a second coordinate valuecharacterizing a differential pressure across the pump for each of aplurality of flow rate data points of a given cardiac cycle of thepatient, each flow rate data point indicative of a flow rate of bloodthrough the pump. An area enclosed by the first and second coordinatevalues of the plurality of flow rate data points is determined, thedetermined area being indicative of an amount of work available to beperformed by the blood pump. The speed of the motor is increased to asecond speed. At the second speed, a first coordinate valuecharacterizing a volume of blood impelled in the pump and a secondcoordinate value characterizing a differential pressure across the pumpfor each of a plurality of flow rate data points of a given cardiaccycle of the patient is calculated, each flow rate data point indicativeof a flow rate of blood through the pump. An area enclosed by the firstand second coordinate values of the plurality of flow rate data pointsis determined, the determined area being indicative of an amount of workavailable to be performed by the blood pump.

In another aspect of this embodiment, the method further includescalculating a difference between the determined area at the first speedand the determined at the second speed, a smaller difference beingindicative of a greater likelihood of a blockage in the blood pump.

In another aspect of this embodiment, a calculated difference of zero isindicative of a total blockage in the blood pump.

In another aspect of this embodiment, the method further includesdetermining whether the determined area at the first speed is at leastone of greater and less than the determined area at the second speed,wherein the determined area at the first speed being less than thedetermined area at the second speed being indicative of a suctioncondition at the blood pump.

In another aspect of this embodiment, the method further includescomparing the determined area to a predetermined value indicative of aminimum allowable amount of ventricular loading, wherein the determinedarea being less than the predetermined value is indicative of thepresence of said adverse condition.

In yet another embodiment, a method of detecting a full-support bloodpump implanted in a patient transitioning to a partial-support stateincludes decreasing a speed of operation of the blood pump. At thedecreased speed, a first coordinate value characterizing a volume ofblood impelled in the pump and a second coordinate value characterizinga differential pressure across the pump for each of a plurality of flowrate data points of a given cardiac cycle of the patient is calculated,each flow rate data point indicative of a flow rate of blood through thepump. An area enclosed by the first and second coordinate values of theplurality of flow rate data points is determined, the determined areabeing indicative of an amount of work available to be performed by theblood pump. The determined area is compared to a predetermined valueindicative of a minimum amount of available work for which the bloodpump enters a partial-support state. When the calculated area is lessthan the predetermined value, further decreasing the speed of operationof the pump, the method is repeatedly performed until the calculatedarea is greater than the predetermined value.

In another aspect of this embodiment, the method further includes at acontrol circuit, detecting the full-support blood pump transitioning tothe partial-support state. Operation of the blood pump at the partialsupport state for at least one cardiac cycle is maintained. After the atleast one cardiac cycle, a speed of operation is increased to at leastone of an original and default speed of operation.

In yet another embodiment, a control circuit for estimating an amount ofwork available to be performed by a blood pump implanted in a patientincludes a flow rate determination circuit configured to repeatedlydetermine flow rate data points, each flow rate data point indicative ofa flow rate of blood through the pump. A volume determination circuitconfigured to calculate, for each determined flow rate data point of agiven cardiac cycle of the patient, a first coordinate valuecharacterizing a volume of blood impelled by the pump is included. Apressure head determination circuit configured is included to calculate,for each determined flow rate data point of the given cardiac cycle, asecond coordinate value characterizing a pressure head exerted by thepump. A pump work determination circuit configured to calculate an areaenclosed by the first and second coordinate values of the flow rate datapoints of the given cardiac cycle is included, the calculated area isindicative of an amount of work available to be performed by the bloodpump.

In another aspect of this embodiment, a flow rate average trackingcircuit is configured to track a running average of the determined flowrate data points and a cardiac cycle determination circuit is configuredto determine a beginning flow rate data point and an end flow rate datapoint of the given cardiac cycle of the patient, each of the beginningand end flow rate data points corresponding to a crossing of thedetermined flow rate data points over the running average.

In another aspect of this embodiment, the cardiac cycle determinationcircuit is configured to determine the beginning and end flow rate datapoints based on at least one from the group consisting of: anidentification of consecutive instances of the determined flow rate datapoints having a negative slope and crossing the running average; anidentification of consecutive instances of the determined flow rate datapoints having a positive slope and crossing the running average; and anidentification of three consecutive instances of the determined flowrate data points crossing of the running average, wherein the first ofthe three consecutive instances corresponds to the beginning flow ratedata point and the third of the three consecutive instances correspondsto the end flow rate data point.

In another aspect of this embodiment, for each determined flow rate datapoint of a given cardiac cycle of the patient, the volume determinationcircuit is configured to calculate the first coordinate value using aderivative of the flow rate data point.

In another aspect of this embodiment, a speed determination circuit fordetermining a rotational speed of the blood pump is included, wherein,for each determined flow rate data point of the given cardiac cycle, thepressure head determination circuit is configured to calculate thesecond coordinate value using the flow rate data point and thedetermined rotational speed of the blood pump.

In another aspect of this embodiment, a memory configured to store areference file containing a correlation between flow rate and pressurehead of the pump for a given operational speed is included, wherein thepressure head determination circuit is operable to determine the secondcoordinate value using interpolation based on the reference file.

In another aspect of this embodiment, the pump includes a housing havingan axis, and a rotor disposed within the housing, the rotor beingrotatable around the axis.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention, and theattendant advantages and features thereof, will be more readilyunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings wherein:

FIG. 1 is a diagram of a pressure-volume loop associated with the leftventricle of a patient;

FIG. 2 is an exploded perspective view of an example blood pump systemin accordance with an aspect of the disclosure;

FIG. 3 is a block diagram of an example control circuit of the bloodpump system of FIG. 2, in accordance with an aspect of the disclosure;

FIG. 4 is a graphical plot of flow rate of blood through a blood pumpover time, in accordance with an aspect of the disclosure;

FIG. 5 is a graphical plot of a measure of volume and a measure ofpressure of a blood pump in accordance with an aspect of the disclosure;

FIG. 6 is a graphical plot of differential pressure as a function offlow in accordance with an aspect of the disclosure;

FIG. 7 is a flow diagram of an example method for determining a pumpcapacity work index in accordance with an aspect of the disclosure;

FIG. 8 is a flow diagram of an example method for identifying a singlecardiac cycle based on flow rate data, in accordance with an aspect ofthe disclosure;

FIG. 9A, is a graphical plot of a measure of volume and a measure ofpressure of a blood pump operating at a speed of 2,000 RPM;

FIG. 9B is a graphical plot of left ventricular and aortic pressure of apatient using a blood pump operating at a speed of 2,000 RPM;

FIG. 10A, is a graphical plot of a measure of volume and a measure ofpressure of a blood pump operating at a speed of 2,200 RPM;

FIG. 10B is a graphical plot of left ventricular and aortic pressure ofa patient using a blood pump operating at a speed of 2,200 RPM;

FIG. 11A, is a graphical plot of a measure of volume and a measure ofpressure of a blood pump operating at a speed of 2,400 RPM;

FIG. 11B is a graphical plot of left ventricular and aortic pressure ofa patient using a blood pump operating at a speed of 2,400 RPM;

FIG. 12A, is a graphical plot of a measure of volume and a measure ofpressure of a blood pump operating at a speed of 2,600 RPM;

FIG. 12B is a graphical plot of left ventricular and aortic pressure ofa patient using a blood pump operating at a speed of 2,600 RPM;

FIG. 13A, is a graphical plot of a measure of volume and a measure ofpressure of a blood pump operating at a speed of 2,800 RPM;

FIG. 13B is a graphical plot of left ventricular and aortic pressure ofa patient using a blood pump operating at a speed of 2,800 RPM;

FIG. 14A, is a graphical plot of a measure of volume and a measure ofpressure of a blood pump operating at a speed of 3,000 RPM; and

FIG. 14B is a graphical plot of left ventricular and aortic pressure ofa patient using a blood pump operating at a speed of 3,000 RPM.

DETAILED DESCRIPTION

Referring now to the drawings in which like reference designators referto like elements, there is shown in FIG. 2 an example blood pump system100 in accordance with one embodiment of the invention. The blood pumpsystem 100 according to this embodiment includes a control circuit 140(not shown) connected via a cable feed 150 to a centrifugal blood pump101. The blood pump 101 includes a housing 105 consisting ofinterlocking casings to form a closed pumping chamber 103 between them.Blood is supplied to the pump 101 through an axial inlet cannula 107adapted for apical insertion into a heart ventricle of a human or animalpatient. The cannula 107 is affixed to or may be integral with thehousing 105 and is in fluid flow communication with the pumping chamber103. Blood exits the pumping chamber 103 through an outlet 113 oppositethe inlet cannula 107 in a direction substantially perpendicular to thelongitudinal axis of the inlet cannula 107.

A motor rotor or pump impeller 122 is located within the pumping chamber103. In operation, blood entering the cannula 107 from a heart ventriclepasses into the pumping chamber 103 where it is engaged by the rotatingimpeller 122. Blood entering the pumping chamber from the cannula 107 isredirected from axial flow exiting the cannula to a radial flow withinwhich the impeller 122 is submerged. Although the example pump 101 ofFIG. 1 is a radial flow blood pump, other types of pumps (e.g., axialflow pumps) are similarly applicable to the present disclosure.

The housing 105 of the pump may contain an electrical feed throughconnector 130 for a power and control cable to supply power to theelectrical motor of the pump. The cable feed 150 carrying a plurality ofcables is connected to the pump through the connector 130. The cables inthe feed 150 may carry electrical power and control instructions to thepump 101.

The control circuit 140 monitors and further controls operation of thepump 101. The control circuit functions may be implemented at least inpart by a general-purpose processor, as shown in the exampleimplementation of FIG. 3. As shown, an example control circuit 201(which may be used as the control circuit 140 of FIG. 2) is implementedusing a processor 210, a memory 220 and an interface 260. Memory 220stores information accessible by processor 210, including instructions250 that may be executed by the processor 210. The memory also includesdata 230 that may be retrieved, manipulated or stored by the processor210. The memory may be of any type capable of storing informationaccessible by the processor, such as a hard-drive, memory card, ROM,RAM, DVD, CD-ROM, write-capable, and read-only memories. The processor210 may be any well-known processor, such as commercially availableprocessors. Alternatively, the processor may be a dedicated controllersuch as an ASIC.

Data 230 may be retrieved, stored or modified by processor 210 inaccordance with the instructions 250. The data may also be formatted inany computer-readable format such as, but not limited to, binary values,ASCII or Unicode. Moreover, the data may comprise any informationsufficient to identify the relevant information, such as numbers,descriptive text, proprietary codes, pointers, references to data storedin other memories (including other network locations) or informationthat is used by a function to calculate the relevant data.

The control circuit 140 includes hardware and software for controllingthe various aspects of the operation of the pump. The control circuit iscoupled to the pump and it is operable to collect at least some of data230 from the pump. For example, data 230 may include pump speed data231, indicating a speed of rotation of the pump's rotor. Pump data 230may also include flow rate data 232 indicative of a flow rate of bloodbeing pushed through the pump (e.g., exiting the pump). As explained ingreater detail in commonly owned published Patent ApplicationPublication Nos. 2012/0245681, 2014/0100413 and 2014/0357937, as well aspending patent application Ser. No. 14/950,467, the disclosures of whichare hereby incorporated herein by reference in their entirety, the flowrate data 232 may be acquired using a model for the estimation of bloodflow rate. In one example, the model determines blood flow rate based inpart on the acceleration of the rotor of the pump and possibly theviscosity of the patient's blood (e.g., based on hematocrit levels).Using such a model results in the estimate having a dynamic range ofabout 15 Hz.

In alternative embodiments, the data 230 may include further informationto estimate blood flow through the pump. For example, in a centrifugalpump, the relationship between the operating current and blood flow ismonotonic for the range of electrical current at which the pump mayoperate. Therefore, the blood flow estimate may be determined by use ofa flow-to-current correlation table.

Similarly, in an axial pump, one or more flow-to-current tables may beused to estimate the blood flow rate based at least in part on ameasured electrical current used to drive the pump. As explained ingreater detail in commonly owned U.S. Patent Publication No.2012/0245681, such estimates may be determined based further on thegiven rotor speed of the pump, a back electromotive force (BEMF) inducedby the impeller on the coils of the rotor, and possibly the viscosity ofthe patient's blood. The estimate of blood flow may be further based atleast in part on the acceleration of the rotor of the pump. Flowestimates have a dynamic range of about 15 Hz.

Additionally, different calculations and parameters may be employed toestimate a flow rate of blood. For instance, blood flow rate may beestimated algorithmically based at least in part on an operatingelectrical current of the pump and a predetermined hematocrit level ofthe blood.

In other examples, flow rate data may be collected based on otherparameters indicative of flow rate. Alternatively, flow rate data may begathered using direct measurements, such as with an ultrasonic flowmeter mounted within the pump.

Data 230 may further include one or more HQ curves 233, correlating aflow rate “Q” with a differential pressure (or differential head) “H”exerted by the pump. In the case of the present disclosure, the HQcurves 233 may indicate, an expected differential pressure exerted bythe pump for a given flow rate of blood exiting the pump. Because therelationship between flow rate and differential pressure varies based onpump speed, different HQ curves 233 may be stored for multiple pumpspeeds, preferably the speeds at which the pump operates. As explainedin greater detail below, the HQ curves 233 may be used to determine adifferential pressure across the pump based on measured, estimated,calculated, or otherwise determined flow rate data 233.

FIG. 6 shows an example set of HQ curves plotting the differentialpressure as a function of flow in a given pump for a plurality ofoperating speeds of the pump. These curves may be predetermined duringdevelopment of the pump. As shown in FIG. 6, the curves may expressdifferential pressure as a function of flow rate for a given motor speedof a given pump. Thus differential pressure across the pump may bedetermined based on the determined flow rate, and further based on athen-current motor speed. Generally, and regardless of motor speed,differential pressure is inversely proportionate to flow rate, such thathigh flow yields lower pressure head and low flow results in higherpressure head, although the inverse relationship is often non-linear. Inthe example of FIG. 6, a flow rate of 5 liters per minute (LPM) (“1”)for a given known pump speed (“2”) is shown to correspond to a pressurehead of about 75 mmHg (“3”) exerted by the pump.

Although the HQ curves of FIG. 6 are shown as a graphical plot, the“curves” stored in memory 220 may be stored in a different format. Forinstance, memory 220 may store a look-up table correlating given flowrates at a given pump speed to respective differential pressures. Then,for a given determined flow rate at a given determined pump speed, thecorresponding differential pressure may be looked up in the table.Corresponding differential pressures for flow rate values not stored inthe look up table may be interpolated based on the values that arestored in the table using approximations, curve fitting and/or othertechniques. Alternatively, memory 220 may store an equation in whicheach of speed, flow rate and differential pressure are variables of theequation. Then, for given values of any two of the variables, the thirdvariable may be calculated based on the equation.

Data 230 may yet further include pump capacity work index (PCWI) values234. As explained in greater detail herein, the PCWI values are valuescharacterizing an amount of work available for the pump to perform for agiven one or more cardiac cycles. PCWI values 234, and particularly howthey are calculated, are explained in greater detail in connection withFIGS. 4 and 5. In many cases, PCWI values may be displayed or otherwiseoutput by an output device 270 coupled to the control circuit 201. Theoutput may then be read by a clinician, who in turn may determinewhether to adjust operation of the blood pump based on those PCWI values234.

Optionally, if a control circuit 201 is programmed to control operationof a pump in response to calculated PCWI values 234, the data 230 mayfurther include prestored PCWI value limits 235. These prestored,predetermined limits may be used in comparisons against the calculatedPCWI values in order to draw conclusions about a given patient's health,and may be good indicators of how a patient should be treated in view ofthe calculated PCWI value 234 (e.g., whether it is undesirable toincrease motor speed of the patient's pump). The PCWI value limits 235may be different for different pumps and different patients, and may bepreset on a patient-by-patient basis. Alternatively, the limits 235 maybe set identically for all similar pumps, thereby setting standard orbaseline values for guiding or otherwise controlling operation of thosepumps.

The instructions 240 stored in the memory 220 may include one or moreinstruction sets or modules for performing certain operations inaccordance with the present disclosure. One such module may be a flowestimation module 241 for performing the steps required to determine aflow rate of blood through the pump. Another such module may be acardiac cycle determination module 242 for performing the steps requiredto determine a beginning point and ending point for a single cardiaccycle (or discrete number of cardiac cycles). A further such module maybe a volume component determination module 243 for determining thevolume component of the available pump work, and a pressure componentdetermination module 244 for determining the pressure component of theavailable pump work. The instructions 240 may also include a pump workdetermination module 255 for calculating a PCWI value 234 based on thedetermined volume and pressure components of modules 243 and 244. Anexample PCWI value calculation is described in greater detail below.

The control circuit 201 may optionally include an interface 250 whichconnects the control circuit 201 to an output device 270. The interface250 may be an analog interface (e.g., audio interface) or a digitalinterface, such as Bluetooth. TCP/IP, wi-fi, and others. Where thecontrol circuit is implemented in an implantable structure adapted to bedisposed within the body of the patient, the interface 250 may includeknown elements for communicating signals through the skin of thepatient. The output device 270, may be a speaker, a light, acommunications terminal (e.g., computer, cell phone), or any other typeof device.

Although FIG. 2 functionally illustrates the processor and memory asbeing within the same block, it will be understood that the processorand memory may actually comprise multiple processors and memories thatmay or may not be stored within the same physical housing. The memorymay include one or more media on which information can be stored.Preferably, the medium holding the instructions retains the instructionsin non-transitory form. Some or all of the instructions and data may bestored in a location physically remote from, yet still accessible by,the processor. Similarly, the processor may actually comprise acollection of processors which may or may not operate in parallel.

An example operation of the control circuit 201 is illustrated by way ofthe graphical representations of FIGS. 4 and 5. FIG. 4 is a graphicalplot of an instantaneous flow rate of blood through a blood pump overtime. Shown in FIG. 4 are about 95 flow rate data points which form aquasi-sinusoidal wave (“Waveform”). Each flow rate data point representsa then-instant flow rate of blood exiting a blood pump. In this example,the blood pump is operating in parallel with the patient's heart. Theflow rate through the pump increases during systole, because the inletof the pump is exposed to the pressure exerted by the heart as itcontracts. Conversely, the flow rate decreases during diastole. Thus,flow rate varies over the course of a cardiac cycle, such that a fullperiod of the sinusoid corresponds to a single cardiac cycle.

Also shown in FIG. 4 are a corresponding number of average flow ratedata points that form another quasi-sinusoidal wave (“Tracker”). In theexample of FIG. 4, the average flow rate data points are based on arunning average, which averages a predetermined number of previouslydetermined flow rate data points, or the flow rate data pointspreviously collected over a predetermined window of time. As shown inFIG. 4, the running flow rate average also varies over time, althoughnot by as much as the then-instant flow rate data.

As shown in FIG. 4, a single cardiac cycle may begin at point “1”(corresponding to data point s37), in which flow rate is decreasing, andhas just dropped below the average flow rate. This indicates the end ofsystole for a previous cardiac cycle and beginning of diastole for thenext cardiac cycle. Point “2” (corresponding to data point s41)indicates a time at which flow rate begins to increase, but is stillless than the average flow rate. At point “3” (corresponding to datapoint s56), flow rate continues to increase and has just increased abovethe average flow rate, indicating the end of diastole and beginning ofsystole of the current cardiac cycle. At point “4” (corresponding todata point s66), flow rate begins to decrease, but remains greater thanaverage flow rate. Lastly, at point “5” (corresponding to data points74) flow rate continues to decrease and drops below the tracked averageflow rate, indicating the end of systole for the current cardiac cycleand beginning of diastole for the next cardiac cycle. In this manner,points “1” and “5” may be considered to mark the beginning and end of asingle cardiac cycle. In other examples, a cardiac cycle may beconsidered to “begin” and “end” at a different phase of the cycle (e.g.,end of diastole, peak flow, minimum flow, etc.), so long as thebeginning and ending mark similar phases of a complete cycle. Points “2”through “4” may further be used to keep track of progression of thecycle, so that it is clear that a full cycle has been completed.

FIG. 5 is a graphical plot of an example PCWI curve based on thewaveform of FIG. 4. Each of the data points shown in FIG. 5 correspondsto a data point (s37 through s74) of the waveform of FIG. 4. Each datapoint is shown in FIG. 4 as being plotted along each of a horizontalaxis (volume component) and a vertical axis (pressure component), muchthe same way a pressure-volume loop is conventionally drawn. In theexample of FIG. 4, the volume component is a second derivative of thevolume of blood exiting by the pump, i.e., the first derivative of theflow rate through the pump over time. Stated another way, the volumecomponent dQ/dt is the slope of the waveform of FIG. 4 at the time ofthe given data point. Also, in the example of FIG. 4, the pressurecomponent is a differential pressure across the blood pump (or pressurehead exerted by the pump) at the time of the given data point. Asexplained above and shown in FIG. 6, differential pressure and/orpressure head may be expressed as a function of flow rate for a givenmotor speed of a given pump.

As an example, FIG. 4 shows that each of data points s37 and s74 has adecreasing slope and an average flow rate, corresponding to a negativederivative of flow rate and midrange differential pressure. Similarly,data point s41 corresponds to a maximum flow, at which the slope of theflow waveform is about 0, and differential pressure is at a minimum.Data point s56 has an increasing slope and an average flow rate,corresponding to a positive derivative of flow rate and midrangedifferential pressure. And data point s66 corresponds to a minimum flow,at which the slope of the flow waveform is about 0, and differentialpressure is at a maximum.

Plotting each flow rate data point on the volume-pressure axes of FIG. 5effectively converts the flow rate data points into PCWI data points,also referred to herein as PCWI coordinates. The PCWI data points may beconsidered to enclose an area of the pressure component/volume componentcoordinate space of FIG. 5. This is illustrated in FIG. 5 by connectingPCWI data points in the time-based order of their corresponding flowrate data points to form a curve. Under normal operation of the pump,the curve may be expected to look something like the curve in FIG. 5,with cyclically increasing and decreasing flow rate derivative, and acyclically increasing and decreasing differential pressure out of phasewith the changes in flow derivative by about 90 degrees (thereby formingan enclosed area). The area of the enclosed space may be determined byany conventional means for estimating, calculating or otherwisedetermining the area of a polygon.

The example systems (control circuits and/or processors) described abovemay be operable to determine PCWI values, as demonstrated above, usingthe operations of the example methods described herein. It should beunderstood that the following operations do not have to be performed inthe precise order described below. Rather, various operations can behandled in a different order or simultaneously. It should also beunderstood that these operations do not have to be performed all atonce. For instance, some operations may be performed separately fromother operations. Moreover, operations may be added or omitted.

FIG. 7 is a flow diagram of a routine 700 for determining a PCWI value.Routine 700 begins at 710 with, for each of the flow rate data points ofa given cardiac cycle, determination of a first coordinate value. Thefirst coordinate value may be a characterization of a volume of bloodimpelled by the blood pump, such as a derivative of a flow rate of bloodthrough the pump (dQ/dt). At 720, for each of the flow rate data pointsof the given cardiac cycle, a second coordinate value is alsodetermined. The second coordinate value may be a characterization ofpressure exerted by the pump, such as a differential pressure across thepump. The first and second coordinate values may effectively map theflow rate data point to a pressure-volume coordinate plane of the bloodpump. Thus, repeating the coordinate transfer of 710 and 720 for eachflow rate data point of the given cardiac cycle yields a loop orenclosed space that effectively represents a pressure-volume loop of theblood pump. At 730, an area of the enclosed space is calculated. Thecalculated area is the PCWI value.

The routine 700 of FIG. 7 may further involve determining a starting andending point of the given cardiac cycle, such that the operations ofFIG. 7 may begin with the starting point of the given cardiac cycle andend with the ending point thereof. FIG. 8 is a flow diagram of anexample routine 800 for determining a starting point and/or ending pointof a cardiac cycle. At 810, a first flow rate data point is determined.At 820, a flow rate moving average is updated based on the first flowrate data point. At 830, the flow rate data point is compared to themoving average to determine which is greater. If the moving average isgreater, operations revert to 810 with determination of another flowrate data point until a flow rate data point is found to be greater thanthe moving average. If the flow rate data point is found to be greaterthan the moving average, then at 940, a next flow rate data point isdetermined, and at 850 the moving average is updated based on said nextflow rate data point. At 860, said next flow rate data point is comparedto the updated moving average to determine which is less. If the movingaverage is less than the flow rate data point, operations revert to 840with determination of another flow rate data point until a flow ratedata point is found to be less than the moving average. If the flow ratedata point is found to be less than the moving average, indicating thatconsecutive flow rate data points have crossed the moving average with anegative slope, then the beginning or ending time of the cardiac cyclehas been identified, and thus corresponds to the first flow rate datapoint that is found to be less than the moving average. Operations maythen resume with either 710 (for a starting time) or 730 (for an endingtime) of FIG. 7.

The example routine 800 of FIG. 8 particularly demonstrates identifyingstarting and ending times for a cardiac cycle beginning and ending witha transition from systole to diastole. However, similar methods may beemployed to identify starting and ending times for other cardiac cycles.For instance, a cardiac cycle beginning and ending with transition fromdiastole to systole may be identified by looking for a transition frombelow the moving average to above the moving average (stated anotherway, consecutive flow rate data points with a positive slope crossingover the moving average). For further instance, a cardiac cyclebeginning and ending with peak or maximum flow may first look forincreases in flow rate followed by a decrease in flow rate. Similarly, acardiac cycle beginning and ending with minimum flow may first look fordecreases in flow rate followed by an increase in flow rate.

First and second coordinates may be calculated and plotted for each ofthe flow rate data points associated with the starting and ending timescollected for the flow rate data point corresponding to the ending time.In such a case, the ending time flow rate data point may beapproximately equal to the starting time flow rate data point, such thatthat the PCWI loop is closed by connecting each of the PCWI coordinatepoints to one another. Alternatively, if the beginning and ending flowrate data points are not approximately equal, and the PCWI loop is stillnot fully closed, the loop may be closed based on an approximation. As afurther alternative, first and second coordinates may be calculated andplotted for the flow rate data points from the starting time to one datapoint preceding the ending time, such that the PCWI coordinate pointscorresponding to those flow rate data points are connected in a loop. Orfirst and second coordinates may be calculated and plotted for the flowrate data points from one data point after the starting time to theending time, such that the PCWI coordinate points corresponding to thoseflow rate data points are connected in a loop.

In some cases, a beginning and ending of a cardiac cycle may be detectedwithout considering slope of the flow rate data points. For instance, afirst instance of consecutive flow rate data points crossing over themoving average may be identified. In such a case, a second instance ofcrossover may indicate a midpoint of the cardiac cycle, and a thirdinstance of crossover may indicate completion of the cardiac cycle.Alternatively, as described in connection with FIG. 4, the cardiac cyclemay be identified by looking for each of (i) a decrease betweenconsecutive flow rate data points occurring below the moving average,(ii) an increase below the moving average, (iii) an increase above themoving average, (iv) a decrease above the moving average, and finally(v) another decrease below the moving average to complete the cardiaccycle.

As a further alternative, the beginning and end of the cardiac cycle maybe determined based on data other than flow rate data. For instance, EKGdata may be used to identify a cardiac cycle, and the beginning and endof the cardiac cycle may be defined as any points of consecutive cardiaccycles having the same phase within the cycle.

Also, the example of FIG. 8 (as well as in FIG. 4) shows the movingaverage being updated as frequently as flow rate data points arecollected. However, in other examples, the moving average may be updatedwith less frequency (e.g., every other flow rate data point, every thirdflow rate data point, every fifth flow rate data point, etc.) or couldbe updated based on a clock instead of the number of flow rate datapoints collected (e.g., every tenth of a second, every quarter second,etc.).

The example PCWI values described above are particularly beneficialbecause they are based entirely on measurements and determinations thatcan be conducted non-invasively, using instrumentation incorporated intothe implanted blood pump and algorithms and data programmed and storedin electronics coupled to and in communication with the blood pump.

FIGS. 9-14A and 9-14B demonstrate the effectiveness of the above examplePCWI examples, as shown in comparison with invasive measurements. FIG.9A shows a pressure-volume loop (bold line) superimposed over a PCWIcurve (thin line) for a blood pump operating at a speed of 2,000 RPM.The pressure-volume loop may be derived using catheter-based invasivemeasurements, whereas the PCWI curve may be derived exclusively fromnon-invasive measurements. As shown in FIG. 9A, the pressure-volume loopencloses a relatively large area, indicating that the heart isperforming a commensurate amount of work. At the same time, the PCWIcurve has a relatively large area of about 1510 (for units ofmmHg*mL/sec²), indicating that there is more work available for the pumpto perform.

FIG. 9B shows invasive hemodynamic measurements of LVP (black line) andAOP (grey line) for a patient using the blood pump of FIG. 9A. As shownin FIG. 9B, under an operating speed of 2,000 RPM, the LVP and AOPmeasurements cross over one another, indicating that the aortic valve isopen during systole.

FIG. 10A shows another pressure-volume loop (bold line) superimposedover a PCWI curve (thin line), now for the same blood pump operating ata speed of 2,200 RPM. As shown in FIG. 10A, the pump has been sped upand is performing more work, leaving less work for the heart to perform.At the same time, the PCWI curve also has a slightly smaller area, nowof about 1350, indicating that the pump has taken on some of thepreviously available work, and now there is less available work.

FIG. 10B shows invasive hemodynamic measurements of LVP (black line) andAOP (grey line) for a patient using the blood pump of FIG. 10A. As shownin FIG. 10B, under an operating speed of 2,200 RPM, the LVP and AOPmeasurements still cross over one another, indicating that the aorticvalve is open during systole.

FIG. 11A shows yet another pressure-volume loop (bold line) superimposedover a PCWI curve (thin line), now for the same blood pump operating ata speed of 2,400 RPM. As shown in FIG. 11A, the pressure-volume loopencloses an even smaller area than the pressure-volume loop of FIG. 10A,since the pump has been further sped up and is performing even morework, leaving even less work for the heart to perform. Likewise, thePCWI curve encloses an even smaller area, now about 982, indicating thatthe pump has again taken on some of the previously available work,leaving even less potential work.

FIG. 11B shows that, under an operating speed of 2,400 RPM, the LVP andAOP measurements still cross over one another, meaning that the aorticvalve is still open during systole.

In FIG. 12A, the operating speed of the blood pump has been increased to2,600 RPM, causing the pressure-volume loop to further shrink, and inturn causing the PCWI curve, to further shrink to an area of about 398.Notably, in FIG. 12B, LVP and AOP no longer cross over one another,indicating that that 2,600 RPM, the blood pump has taken on so much workthat there is not enough work left at the heart for the heart to forceopen the aortic valve and eject blood from the left ventricle duringsystole.

FIG. 13A further demonstrates the shrinking of the PCWI curvecommensurate with the shrinking of the pressure-volume loop at anoperational speed of 2,800 RPM. Now, the area of the PCWI curve is onlyabout 77.5. As expected, in FIG. 13B, aortic valve closure persists at2,800 RPM, as evidenced by the consistent difference between LVP andAOP.

FIG. 14A demonstrates the even further shrinking of the PCWI curve,again commensurate with the shrinking of the pressure-volume loop, at anoperational speed of 3,000 RPM. Now, the area of the PCWI curve is onlyabout 5.06. As shown in FIG. 14B, not only does the aortic valve closurepersist, but LVP and AOP remain relatively unchanged over the course ofthe cardiac cycle, since there is little change in either flow orpressure at any of the left ventricle, aorta, or pump.

In situations such as that of FIGS. 14A and 14B, there is little benefitto increasing the operating speed any further, since there is virtuallyno work left for the blood pump to perform. Even in situations such asthat of FIGS. 11A and 11B, where increasing operating speed of the bloodpump may result in aortic valve closure, it may also be undesirable toincrease the operating speed. Even operation at a point before aorticvalve closure may be undesirable for a given patient if the patient'sheart is well enough to perform a given amount of work. Since PCWIvalues are a helpful way to characterize the amount of work performed bythe patient's heart, and how much work remains for the pump to perform,the PCWI values may also be a helpful away to characterize a limit ofdesirable amount of pump work for a given patient.

The PCWI value may be used in several applications. For instance, thePCWI value may be compared to a predetermined value setting a limit. Thelimit may indicate a maximum amount of work for which it is desirablefor the pump to perform, or for which partial-support status of the pumpis maintained (e.g., preventing aortic valve closure). When the PCWIvalue is greater than the limit, a clinician may determine that it issafe or even desirable to increase the operating speed of the pump.However, when the PCWI value is less than the limit, the clinician maydetermine that it is undesirable or even unsafe to increase theoperating speed. In such a case, if a patient complains of discomfort tothe clinician, the clinician may at first glance wish to increase thepump speed, but after checking the patient's PCWI limit and the pumps'PCWI value, the clinician may determine that something else is wrongwith the patient (e.g., cardiovascular accident, or other adversecardiac event) for which increasing pump speed may not help, and mayeven hurt, the patient. Stated another way, if the clinician discoversfrom the PCWI value has passed a threshold limit indicative of a minimumamount of ventricular loading by the patient's heart, the clinician mayconclude that the patient must be evaluated to determine the cause ofreduced work performed by the patient's heart. Thus, the PCWI value maybe used to detect aortic valve closures and/or adverse events at thepatient's heart.

In some cases, the predetermined limit may be set on a patient-specificbasis. Alternatively, the predetermined limit may vary on acondition-to-condition basis, and/or may be a standard figure appliedfor multiple patients.

In some cases, the predetermined limit may be stored in the memory of acontrol circuit (e.g., PCWI limit 235) and used to control operation ofthe pump. For instance, a given PCWI limit may be compared to a currentPCWI value, and if the PCWI value is less than the PCWI limit, thecontrol circuit may activate an alert and/or override any attempts tofurther increase operating speed of the pump, or may slow down the pump.

Another application of the PCWI value may be to detect an incipientsuction condition. Specifically, a very low PCWI value (e.g., thoseshown in FIGS. 13A and 14A) indicate that there is little to no changeof dQ/dt at the pump, nor is there much change in differential pressure.This may in turn be indicative that the ventricle of the patient's heartis not filling, and that suction on the ventricle is imminent. In thosecases where a PCWI value indicative of such conditions is programmedinto a control circuit, the control circuit may react to such acalculated PCWI by reducing an operational speed of the pump so as toallow the ventricle an opportunity to fill.

Another application of the PCWI value may be to detect blockages in thepump. For instance, if the pump is operating at a first speed, having acorresponding PCWI value, one would expect the PCWI to decrease with acorresponding increase in speed. However, if the operating speed of thepump is increased and the PCWI value remains unchanged, or if the PCWIvalue only changes by less than a threshold amount, it may indicate thatthe pump is not taking on work from the heart. This could be due to ablockage in the pump. Thus, the lack of change in the PCWI value is atleast indicative of a greater likelihood of a blockage in the pump.

Yet another application of the PCWI value may be to detect a suctioncondition in a partial-support blood pump. For instance, if the pump isoperating at a first speed, having a corresponding PCWI value, one wouldexpect the PCWI to decrease with a corresponding increase in speed.However, under suction conditions, the flow rate of blood sharply dropstowards zero at the beginning of diastole, and then sharply increases,causing a brief but large change in dQ/dt, and a brief but large changein differential pressure. This brief but large change may result in aPCWI curve enclosing a larger area than enclosed prior to suction, whenthe pump was operating under a condition of increasingly low flowpulsatility. Thus, if the operating speed of the pump is increased andthe PCWI value actually increases (e.g., increases by any amount,increases by a threshold amount), it may indicate that the pump hascaused a suction condition at the left ventricle of the patient. Theoperating speed of the pump may be reduced in response to thisdetermination

An even further application of the PWCI value may be to control atemporary transition of a full-assist blood pump to a partial-assistmode. In some cases, it is desirable to slow the operating speed of afull-assist blood pump so that the patient's heart forces open theaortic valve (or pulmonary valve) during systole. Even if the patient'sheart is not healthy enough to be relied on to regularly pump blood,periodically opening the aortic valve may be beneficial to preventdisrepair of the valve itself (e.g., calcification, clotting, etc.). Asexplained above, the PCWI value may provide a good indication of whetherthe patient's heart is performing enough work to open the aortic valve.Thus, if a PCWI value corresponding to that amount of work isdetermined, that value may be used to ensure pump slowdown that causesaortic valve opening, or stated differently, that ensures AOP/LVPcrossover, without having to rely on invasive measurements. If the PCWIvalue is stored in a memory of the pump control circuit, the controlcircuit may provide automated, intermittent slowdown of the pump (andthen resumption of the previous operating speed) in order to temporarilyopen the patient's aortic valve.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended paragraphs.

What is claimed is:
 1. A control circuit for estimating an amount ofwork available to be performed by a blood pump implanted in a patient,comprising: a flow rate determination circuit configured to repeatedlydetermine flow rate data points, each flow rate data point indicative ofa flow rate of blood through the pump; a volume determination circuitconfigured to calculate, for each determined flow rate data point of agiven cardiac cycle of the patient, a first coordinate valuecharacterizing a volume of blood impelled by the pump; a pressure headdetermination circuit configured to calculate, for each determined flowrate data point of the given cardiac cycle, a second coordinate valuecharacterizing a pressure head exerted by the pump; and a pump workdetermination circuit configured to calculate an area enclosed by thefirst and second coordinate values of the flow rate data points of thegiven cardiac cycle, the calculated area is indicative of an amount ofwork available to be performed by the blood pump.
 2. The control circuitof claim 1, further including: a flow rate average tracking circuitconfigured to track a running average of the determined flow rate datapoints; and a cardiac cycle determination circuit configured todetermine a beginning flow rate data point and an end flow rate datapoint of the given cardiac cycle of the patient, each of the beginningand end flow rate data points corresponding to a crossing of thedetermined flow rate data points over the running average.
 3. Thecontrol circuit of claim 2, wherein the cardiac cycle determinationcircuit is configured to determine the beginning and end flow rate datapoints based on at least one from the group consisting of: anidentification of consecutive instances of the determined flow rate datapoints having a negative slope and crossing the running average; anidentification of consecutive instances of the determined flow rate datapoints having a positive slope and crossing the running average; and anidentification of three consecutive instances of the determined flowrate data points crossing of the running average, wherein a first of thethree consecutive instances corresponds to the beginning flow rate datapoint and a third of the three consecutive instances corresponds to theend flow rate data point.
 4. The control circuit of claim 1, wherein,for each determined flow rate data point of a given cardiac cycle of thepatient, the volume determination circuit is configured to calculate thefirst coordinate value using a derivative of the flow rate data point.5. The control circuit of claim 1, further comprising a speeddetermination circuit for determining a rotational speed of the bloodpump, wherein, for each determined flow rate data point of the givencardiac cycle, the pressure head determination circuit is configured tocalculate the second coordinate value using the flow rate data point andthe determined rotational speed of the blood pump.
 6. The controlcircuit of claim 5, further comprising a memory configured to store areference file containing a correlation between flow rate and pressurehead of the pump for a given operational speed, wherein the pressurehead determination circuit is operable to determine the secondcoordinate value using interpolation based on the reference file.
 7. Thecontrol circuit of claim 1, wherein the pump includes a housing havingan axis, and a rotor disposed within the housing, the rotor beingrotatable around the axis.